The in situ variation of dimethyl sulfide (CH3SCH3, DMS) at a fixed station in a coastal area of the Baltic Sea has been studied for a period of time (January 1987 to June 1988) covering the annual biological cycle. DMS in the surface waters of the brackish Baltic Sea showed a clear seasonal variation, ranging from 2 to 200 ng S L -1 . Lowest concentrations were in winter, peak values followed the spring bloom, and a pronounced maximum was found during the summer (July-August). Concentrations above low winter levels occurred only when the trophogenic layer was depleted of inorganic nitrogen. From our data it is clear that the seasonal variation in DMS concentration is related to biological activity. However, we did not find any correlations between DMS concentration and gross parameters such as chlorophyll a, total phytoplankton biomass, or primary production on an annual basis. Further, we were not able to relate high DMS concentrations to any particular phytoplankton species or species assemblages. It appears that DMS production is primarily associated with phytoplankton growth under nitrogen-limited conditions and not with certain species. We found a significant correlation of ambient DMS concentration with copepod and total zooplankton biomass, suggesting zooplankton grazing pressure as the major factor responsible for the liberation of /3-dimethylsulfoniopropionate (DMSP) from phytoplankton cells and thus for the DMS production. The turnover time of DMS in the water column was calculated to be of the order of 2 days, and the most effective sink process seems to be of microbiological and/or chemical origin. Previous to this study, no in situ data set has been available to test the relative importance of the various factors responsible for the DMS production in seawaters. We have demonstrated that variations in DMS concentration must be looked upon as the result of complex physiological as well as ecological interactions. INTRODUCTIONThe ocean surface layer plays an important role in the global biogeochemical sulfur cycle. Dimethyl sulfide (DMS) constitutes about 90% of the reduced volatile sulfur in surface seawaters [Wakeham et al., 1987; Leck and Bggander, 1988] and is far in excess of the concentration expected at equilibrium with atmospheric concentration [Lovelock et al., 1972; Maroulis and Bandy, 1977; Barnard et al., 1982; Cline and Bates, 1983; Dacey and Wakeham, 1984]. This gives rise to a flux of DMS from the ocean to the atmosphere estimated to be in the range 30-50 Tg S a -1 globally [Andreae, !986]. The total natural flux of gaseous sulfur to the atmosphere originating from oceans, continents, and volcanoes is estimated to be around 80 Tg S a -• . This figure is of the same order of magnitude as the anthropogenic sulfur flux from fossil fuel combustion [Cullis and Hirschler, 1980; Mbller, 1984]. In the boundary layer of the marine atmosphere, DMS is photochemically oxidized to non-sea-salt sulfate (NSS-SO42-) via intermediates such as sulfur dioxide (SO2) and methane sulfonic acid (CH3SO3 H) [And...

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